1. Field of the Invention
The present invention relates to a fixing apparatus for use in image forming apparatus such as an electrophotographic copying machine and data recorder. More particularly, the present invention relates to such type of fixing apparatus wherein a nip pressure is applied to a toner image bearing member between a pair of rotating members so as to fix the toner image onto the bearing member.
2. Description of the Prior Art
In the art, there are known and used various types of fixing systems. They are generally classified into two groups, thermal fixing systems and pressure fixing systems. Examples of the former group are heat chamber systems, flash fixing systems and heating roller systems employing rotating members. A typical example of the latter systems is a pressure roller fixing system in which a pressure is applied by a pair of rollers.
According to the thermal fixing method, heat is applied to the toner image to fuse the toner and fix it to the image bearing member. The pressure fixing method uses a pair of rollers and the toner is fixed to the toner image bearing member by the nip pressure between the rollers. The pressure fixing method need not use heat and has an advantage in that there is no wait time as compared with the thermal fixing method. For example, in the case of heating roller system, the fixing apparatus can not be usable until the roller is warmed up to a necessary temperature. The time required for this warming up is mere wait time during which the apparatus is standing at rest. For pressure fixing systems such wait time is unnecessary. For this advantage, in these years, the pressure fixing system has attracted attention in the art.
However, the pressure fixing system involves some problems. Among them, irregularity of pressure is the most important problem. In the pressure fixing apparatus, the pressure application is done at the side plates by means of air pressure or spring pressure. If the transfer sheet (image bearing member) passes through between the pair of rollers in a position deviated from the symmetrical position with respect to the length of the rollers, then the rollers are somewhat inclined and therefore the nip pressure can no longer be applied uniformly to the transfer sheet. The pressure applied to the transfer material is higher than necessary at some part and lower at another part of the transfer material. Such irregularity of applied pressure results in poor image quality.
To solve the problem, there have been proposed and used some methods. One solution hitherto known is to apply a very high pressure to the whole image bearing member so as to obtain the necessary fixing pressure even at the lowest pressure portion. Another solution is to design the fixing apparatus in such manner that all image bearing members always pass through the middle of the roller no matter what different sizes the image bearing members may have. However, for the first mentioned solution, employing very large pressure, it is required to increase the durability and pressure resistance of the apparatus. This makes the apparatus large in size and heavy in weight. The latter mentioned solution also has some difficulties. After transferring the toner image to the image bearing member from the photosensitive medium, it is required to separate the image bearing member from the photosensitive medium with the aid of particular means such as corona discharger or gripper. Without such aid, it is difficult to convey the image bearing material correctly. Therefore, when the latter mentioned solution is used, it becomes very difficult to employ any simple separation method such as a separation belt system. These difficulties result in cost-up and complication of the apparatus.
For better understanding of the present invention, the problem involved in the prior art fixing system will be further described hereinafter with reference to FIG. 1.
FIG. 1A shows a pair of rollers used in the prior art pressure fixing apparatus. FIG. 1B shows the same pair of rollers with a sheet of image bearing member between the rollers by which a developed image is fixed on the sheet. FIG. 1C is a graph showing the distribution of pressure applied to the sheet.
In FIG. 1A, the two rollers R1 and R2 are pressed against each other by a known pressing mechanism (not shown). In the shown position, no fixing is being carried out and the two rollers are in contact with each other under a constant pressure. Therefore, in this position, the distribution of the nip pressure is uniform. In FIG. 1B, P is a sheet of transfer material which is now passing through the nip between the rollers R1 and R2 for fixing. At both ends of the rollers R1, R2 a load (pressure) W1 is being applied to the rollers. The transfer sheet P between the rollers R1 and R2 is deviated from the symmetrical position with respect to the axial direction of the roller. As the center line of the transfer sheet is not in alignment with the middle of the nip width between R1 and R2, the upper roller R1 is somehow inclined as seen in FIG. 1B. By this inclination of the roller, at the axial middle area of the roller, an excessively large pressure is applied to the sheet P. On the contrary, at the axially end area of the roller toward which the sheet is deviated from the center, the roller R1 becomes raised and the pressure applied to the sheet P is less than the necessary fixing pressure. Therefore, in this area, the toner image is insufficiently fixed. Such irregularity of applied pressure to the sheet P is seen best from the pressure distribution curve in FIG. 1C.
The present invention has solved the problem in a very simple manner. With the fixing apparatus according to the invention, the fixing pressure is applied always uniformly to all parts of the image bearing member.